1. Field of the Invention
The invention relates to helical drag bits and rock bolt systems, which can be used for geotech, mining, and excavation purposes. The invention also relates to methods of using such helical drag bits, and systems incorporating such helical drag bits and rock bolts.
2. Related Art
Known drilling systems may employ roller cone bits, which operate by successively crushing rock at the base of a bore. Roller cone bits are disadvantageous because rock is typically resistant to crushing. Other known rock drilling systems employ drag bits. Conventional drag bits operate by shearing rock off at the base of the bore. Drag bits can be more efficient than roller cone bits because rock is typically less resistant to shearing than to crushing.
Most state of the art rock cutting processes are accomplished by the shearing action or grinding motion of some cutting tool. These cutting actions result in a noisy work environment coupled with the undesirable excitation vibrations that are transmitted to the drill unit home structure. A parameter of paramount importance in any drilling process is the “weight-on-bit” which is the axial force acting on the bit during the cutting process. Normally this force is relatively large and may be generated via proper anchoring of the drill machine to the drilled surface or as an alternative, weight-on-bit may be provided by the self-weight of the drill unit structure.
U.S. Pat. No. 5,641,027 to Foster (“the '027 patent”; assigned to UTD Incorporated) discloses a drilling system incorporating a bit with thread cutting members arranged in a helical pattern. Each subsequent cutting member is wedge shaped such that the threads cut by the bit are fragmented, i.e., snapped off. The bit disclosed by the '027 patent is suitable for enlarging a bore formed by a pilot drill bit. The entirety of the '027 patent is hereby incorporated by reference herein.
A Low Reaction Force Drill (LRFD), such as that disclosed in the '027 patent, is a low-energy, low mass, self-advancing drilling system. Energy expenditures have been demonstrated by studies to be at least five times less than other prior art systems suitable for similar drilling purposes. The distinct advantages of the LRFD are its low energy drilling capability as a function of its unique rock cutting mechanism, its essentially unlimited depth capability due to its tethered downhole motor and bailing bucket configuration, its self-advancing capability by self-contained torque and weight-on-bit by counteracting multiple concentric rock cutters and bracing against rock or regolith. Additional LRFD advantages may be found in its large non-thermally degraded intact sample production (>1 cm3) with position known to within 15 mm, and finally, the large diameter hole it produces that allows for down hole instrumentation during and post drilling. The system has application for shallow drilling (1 to 200 meters) through kilometer class drilling in a broad range of materials. It would be advantageous to utilize the advantages of this system in a new drag bit geometry, while also mitigating disadvantageous characteristics of this system with a new bit.
It would be advantageous to have a helical drag bit that utilizes fewer power resources and that can operate with or without fluid lubrication. It would also be advantageous if such a drag bit could operate under extreme cold and near vacuum conditions, such as those found at extra-terrestrial sites.
A problem encountered by geologists or other rock mechanics investigators is the difficulty of obtaining accurate compressive strength measurements of rock in the field, particularly in situ during drilling. In conventional drilling, several drilling variables must be simultaneously monitored in order to interpret lithologic changes, including thrust, rotational velocity, torque, and penetration rate. This is true because with each conventional bit rotation the amount of material removed is a function of all of those variables. It would be advantageous for a geo-technical system to enable geologists and others to obtain accurate substrate characteristic measurements in situ.
In the mining industry, roof falls in coal mines continue to be the greatest safety hazard faced by underground coal mine personnel. The primary support technique used to stabilize rock against such events in coal and hard rock mines are rock bolts or cable bolts. Both of these primary support techniques involve drilling holes in rock and establishing anchoring in those holes. Current fatality and injury records underscore the need to improve these operations.
As the primary means of rock reinforcement against roof collapse, rock bolts play an important role. As collected from rock bolt manufacturers by NIOSH (i.e., the National Institute for Occupational Safety and Health), approximately 100 million rock bolts were used in the U.S. mining industry in 1999 and of those, approximately 80% used grout as a means of anchoring the bolt to the rock (up from approximately 48% in 1991) with the vast majority of the remaining percentage of rock bolts using mechanical anchors. Cuts through mountainous terrains by highways and railways also extensively use rock bolts or cable bolts for rock mass stabilization.
While a broad range of anchoring techniques have been developed, grouting and mechanical expansion anchor bolts are the more common, together comprising over 99% of rock bolts used in coal mines in the U.S. The decline in the use of mechanical bolts is attributed to the fact that grouted rock bolts distribute their anchoring load on the rock over a greater area and generally produce better holding characteristics.
As a major contributor to a roof control plan, rock bolts have been studied to determine optimum installation spacing, length, and matching of anchoring with geologic conditions. The main ways rock bolts support mine roofs are typically described as follows: beam building (the tying together of multiple rock beams so they perform as a larger single beam), suspension of weak fractured ground to more competent layers, pressure arch, and support of discrete blocks. Cable bolting (where cables are used in place of steel rods as bolts) performs similar functions. While rock bolts play a critical role in mitigating rock mass failure, many other mine design factors come into play to create a stable mine environment including (but not limited to) opening dimensions, sequence of excavation, matching of bolt anchor and length with opening and geologic conditions, and installation timing. Notwithstanding the importance of these other factors, if the rock bolts used in rock stabilization do not perform well, miners are at risk.
Bolt installation characteristics near roof falls have been identified as contributing to failure. One documented and regularly occurring rock bolt failure mechanism is loss of grout shear bond to the rock wall of the bolt hole. Key contributors to the integrity of the grout interlocking with the rock mass are the diameter of the hole relative to the diameter of the bolt, resin vs. cement type grouts, rock type and condition of the hole.
Smooth bolt holes consistently produce a reduction in rock bolt load bearing capacity over rough walled holes. To address this, bolt hole bit manufacturers intentionally use reduced tolerances in their manufacturing on the center of bit peaks, and setting of bit cutter inserts in such a way as to induce a wobble during drilling, as well as loose bit mounting to drill rod, with the ultimate result of ridges being left on hole walls. The approach generally produces increased anchoring capacity. However, even with these variations in bolt hole smoothness, anchorage capacity increases, but failure of the rock-grout interface is still common.
While considerable research into rock bolting has been conducted to date, gaps still exist in areas that could lead to vast improvements in rock bolt performance. For example, significant pull-test studies have been performed and optimal hole diameter to bolt diameter ratios have been identified for maximum anchorage capacity, and hole condition has been identified as an important contributor to ultimate holding capacity. A relatively unexplored feature in rock bolt holding capacity is hole geometry. It would be advantageous to optimize bolt hole geometry for improved holding capacity.
Other problems are also encountered in the field of rock bolt hole drilling: dust and noise. During most rock bolt drilling operations, the operator stands directly at the controls, a couple of feet away from the machinery and the actual drilling process. Research by NIOSH has identified potential for high silica dust levels around roof bolters in coal mines and attributes much of the cause to the vacuum collection and filtering of air used in the drilling process. While significant research into dust hazards and health effects has been conducted by NIOSH (and previously by the U.S. Department of Interior, Bureau of Mines), the measures to improve the environment for rock bolt drillers has been limited almost entirely to worker protection actions.
Noise near mining machinery has also been studied. Engineering solutions to the mitigation of high noise levels are always preferred over administrative solutions or personal protective equipment. The key is to make those engineering solutions cost-effective.
Similarly, dust protective equipment is useful, but low-dust-by-design solutions offer greater opportunity for seamless incorporation and effectiveness in improving the safety and health environment for miners.